1. Technical Field
The present invention relates to an illumination device and a projector.
2. Related Art
Liquid crystal projectors are devices of modulating the light emitted from an illumination device in accordance with image information using a liquid crystal device, and then projecting the image thus obtained in an enlarged manner using a projection lens.
In recent years, as such liquid crystal projectors, extremely small projectors (so-called pico projectors) aiming at installation into portable equipment such as cellular phones or digital cameras have been under development.
Here, in a small-sized projector, it becomes necessary to simplify the configuration of the power supply circuit or the optical system, or to miniaturize these constituents.
As such a small-sized projector, there has been known a device having, for example, an illumination device composed of a solid-state light source device, a collimator lens unit, and a polarization conversion element, a liquid crystal panel as a light modulation device, and a projection optical system.
In this projector, since a single liquid crystal panel is used as the light modulation device, and further, the optical members such as integrator optical system, which is typically disposed between the light source and the polarization conversion element, are eliminated, it is possible to achieve reduction in size.
Incidentally, in the liquid crystal projectors, those arranged to separate the light from the collimator lens into a P-polarized light component and an S-polarized light component, and further convert, for example, the P-polarized light component into the S-polarized light component are used as the polarization conversion element (see, e.g., JP-A-2010-72138). An example of such a polarization conversion element is shown in FIG. 12.
The polarization conversion element 200 shown in FIG. 12 is for converting the light L input from a collimator lens unit not shown into the light composed of an S-polarized light component. The polarization conversion element 200 is composed of an optical block 200A and an optical block 200B. The configuration of the optical block 200A is the same as the configuration of the optical block 200B, and the optical block 200A and the optical block 200B are each composed of a polarization beam splitter 201 provided with a polarization splitting film 203, and an internal total reflection prism 202 provided with a total reflection film 204. Further, the optical block 200A and the optical block 200B are disposed symmetrically about an illumination light axis 200ax. Therefore, in the following explanation, only the optical block 200A will be explained, and the explanation related to the optical block 200B will be omitted.
Among the plurality of surfaces provided to the polarization conversion element 200, the surface on the side to which the light L emitted from the collimator lens unit is input is referred to as a light entrance surface S21 of the polarization conversion element 200. In the light entrance surface S21, the region corresponding to the light entrance surface of the polarization beam splitter 201 constitutes a light guide port B3 through which the light L is introduced into the polarization conversion element 200.
Further, among the plurality of surfaces provided to the polarization conversion element 200, the surface opposed to the light entrance surface of the polarization conversion element 200 is referred to as a light exit surface of the polarization conversion element 200. In the light exit surface S22 of the polarization conversion element 200, the region corresponding to the light exit surface of the polarization beam splitter 201 is referred to as a “first region B1.” Further, in the light exit surface S22 of the polarization conversion element 200, the region corresponding to the light exit surface of the internal total reflection prism 202 is referred to as a second region B2.” The first regions B1 are provided with a wave plate 206 for converting the P-polarized light component into the S-polarized light component.
The tilted surface located inside the polarization beam splitter 201 is provided with the polarization splitting film 203 for reflecting the S-polarized light component, which is input from the light guide port B3 of the light entrance surface S21, in a direction perpendicular to the illumination light axis 200ax, and at the same time transmitting the P-polarized light input from the light guide port B3. Further, the tilted surface located inside the internal total reflection prism 202 is provided with a total reflection film 204 for reflecting the S-polarized light component, which is reflected by the polarization splitting film 203, in a direction parallel to the illumination light axis 200ax. 
In such a polarization conversion element 200, the light L emitted from the collimator lens unit proceeds to the inside of the polarization conversion element 200 through the light guide port B3, and then enters mainly the polarization splitting film 203. In the light having entered the polarization splitting film 203, the S-polarized light component with respect to the polarization splitting film 203 is reflected by the polarization splitting film 203 and the total reflection film 204, and thus the light path thereof is translated from the polarization beam splitter 201 toward the internal total reflection prism 202. Then, the S-polarized light component is emitted from the second region B2 as the illumination light. On the other hand, in the light having entered the polarization splitting film 203, the P-polarized light component with respect to the polarization splitting film 203 is transmitted through the polarization splitting film 203. The P-polarized light component having been transmitted through the polarization splitting film 203 is transmitted through the wave plate 206 to thereby be converted into the S-polarized light component, and is then emitted from the first region B1 as the illumination light.
However, in the configuration of inputting the light, which is emitted from the collimator lens unit, to the polarization conversion element 200, there arises the following problem.
That is, since the light from the collimator lens is collimated light, although the light emitted from the inner peripheral area of the collimator lens goes straight, and then enters the light guide port B3, the light emitted from the outer peripheral area of the collimator lens enters the outside of the light guide port B3, and is therefore reflected by the total reflection film 204 to the outside of the polarization conversion element 200. Therefore, there arises a problem that the use efficiency of the source light is lowered, which makes it difficult to increase the intensity of the illumination light. Therefore, it has been proposed that the reflecting film is disposed in the area outside the light guide port B3 of the light entrance surface S21, and the light having entered the area is reflected toward the solid-state light source device to thereby be reused as the source light. However, the effect thereof is not so significant.
Further, in such a configuration, although the illumination light emitted from the first region B1 is the light having proceeded straight while being transmitted through the polarization splitting film 203 and the wave plate 206, the illumination light emitted from the second region B2 is the light having passed through the reflection path from the polarization splitting film 203 toward the total reflection film 204. Therefore, the light path length of the illumination light emitted from the second region B2 corresponds to what is obtained by adding the length of the light path corresponding to the reflection path to the length of the light path through which the illumination light from the first region B1 has passed. Therefore, according to the law of illumination, the intensity of the illumination light emitted from the second region B2 is lower than the intensity of the illumination light emitted from the first region B1, and there arises a problem that there occurs the illuminance unevenness that the area irradiated with the light from the second region is darker than the area irradiated with the light from the first region.